Musannaf Ibn Abi Shayba 33:215

Narrated to us Abd al-Rahman ibn Muhammad al-Muharibi, from Malik, from Abu al-Safar, who said: Some of his brethren entered upon Abu Bakr to visit him during his illness. They said: "O Caliph of the Messenger of Allah, may Allah bless him and grant him peace, shall we not call a physician to examine you?" He said: "He has already examined me." They said: "And what did he say to you?" He said: "He said: 'Indeed, I am one who does what I intend.'"